Sodium hydroxide is classified as a strong base because in water it:

1. The most common bases are ionic compounds of group 1 or group 2 cations with the hydroxide ion. 2. These compounds do not react chemically with water; instead they dissolve and dissociate, releasing hydroxide ions directly into the solution. 3. Since the dissociation process is essentially complete when ionic compounds dissolve in water under typical conditions, sodium hydroxide and other ionic hydroxides are all classified as strong bases. 4. A base that reacts only partially is classified as weak instead. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 4 'Stoichiometry of Chemical Reactions', sections 4.1-4.5_